Membrane probing system

ABSTRACT

A membrane probing assembly includes a probe card with conductors supported thereon, wherein the conductors include at least a signal conductor located between a pair of spaced apart guard conductors. A membrane assembly includes a membrane with contacts thereon, and supporting at least a signal conductor located between a pair of spaced apart guard conductors. The guard conductors of the probe card are electrically interconnected proximate the interconnection between the probe card and the membrane assembly. The guard conductors of the membrane assembly are electrically interconnected proximate the interconnection between the probe card and the membrane assembly.

BACKGROUND OF THE INVENTION

The present invention relates to probe assemblies of the type commonlyused for testing integrated circuits (IC).

The trend in electronic production has been toward increasingly smallergeometries particularly in integrated circuit technology wherein a verylarge number of discrete circuit elements are fabricated on a singlesubstrate or “wafer.” After fabrication, this wafer is divided into anumber of rectangular-shaped chips or “dice” where each die presents arectangular or other regular arrangement of metallized contact padsthrough which input/output connections are made. Although each die iseventually packaged separately, for efficiency sake, testing of thecircuit formed on each die is preferably performed while the dies arestill joined together on the wafer. One typical procedure is to supportthe wafer on a flat stage or “chuck” and to move the wafer in X, Y and Zdirections relative to the head of the probing assembly so that thecontacts on the probing assembly move from die to die for consecutiveengagement with each die. Respective signal, power and ground lines arerun to the probing assembly from the test instrumentation thus enablingeach circuit to be sequentially connected to the test instrumentation.

One conventional type of probing assembly used for testing integratedcircuits provides contacts that are configured as needle-like tips.These tips are mounted about a central opening formed in a probe card soas to radially converge inwardly and downwardly through the opening.When the wafer is raised beyond that point where the pads on the waferfirst come into contact with these tips, the tips flex upwardly so as toskate forwardly across their respective pads thereby removing oxidebuildup on the pads.

The problem with this type of probing assembly is that the needle-liketips, due to their narrow geometry, exhibit high inductance so thatsignal distortion is large in high frequency measurements made throughthese tips. Also, these tips can act in the manner of a planing tool asthey wipe across their respective pads, thereby leading to excessive paddamage. This problem is magnified to the extent that the probe tips bendout of shape during use or otherwise fail to terminate in a common planewhich causes the more forward, ones of the tips to bear down too heavilyon their respective pads. Also, it is impractical to mount these tips atless than 100 micron center-to-center spacing or in a multi-rowgrid-like pattern so as to accommodate the pad arrangement of moremodern, higher density dies. Also, this type of probing assembly has ascrub length of the needle tips of 25 microns or more, which increasesthe difficulty of staying within the allowed probing area.

In order to reduce inductive losses, decrease pad wear, and accommodatesmaller device geometries, a second type of probing assembly has beendeveloped that uses a flexible membrane structure for supporting theprobing contacts. In this assembly, lead lines of well-defined geometryare formed on one or more plies of flexible insulative film, such aspolyimide or MYLAR™. If separate plies are used, these plies are bondedtogether to form, for example, a multilayered transmission linestructure. In the central portion of this flexible structure ormembrane, each conductive line is terminated by a respective probingcontact which is formed on, and projects outwardly from, an outer faceof the membrane. These probing contacts are arranged in a predeterminedpattern that matches the pattern of the device pads and typically areformed as upraised bumps for probing the flat surfaces conventionallydefined by the pads. The inner face of the membrane is supported on asupporting structure. This structure can take the form, for example, ofa truncated pyramid, in which case the inner face of the center portionof the membrane is supported on the truncated end of the support whilethe marginal portions of the membrane are drawn away from the centerportion at an angle thereto so as to clear any upright components thatmay surround the pads on the device.

With respect to the membrane probing assembly just described, excessiveline inductance is eliminated by carefully selecting the geometry of thelead lines, and a photolithographic process is preferably used to enablesome control over the size, spacing, and arrangement, of the probingcontacts so as to accommodate higher density configurations. However,although several different forms of this probing assembly have beenproposed, difficulties have been encountered in connection with thistype of assembly in reducing pad wear and in achieving reliable clearingof the oxide layer from each of the device pads so as to ensure adequateelectrical connection between the assembly and the device-under-test.

One conventional form of membrane probing assembly, for example, isexemplified by the device shown in Rath European Patent Pub. No.259,163A2. This device has the central portion of the sheet-likemembrane mounted directly against a rigid support. This rigid support,in turn, is connected by a resilient member comprising an elastomeric orrubber block to the main body of the assembly so that the membrane cantilt to match the tilt of the device. Huff U.S. Pat. No. 4,918,383 showsa closely related device wherein radially extending leaf springs permitvertical axis movement of the rigid support while preventing it fromtilting so that there is no slippage or “misalignment” of the contactbumps on the pads and further so that the entire membrane will shiftslightly in the horizontal plane to allow the contacts to “scrub” acrosstheir respective pads in order to clear surface oxides from these pads.

In respect to both of these devices, however, because of manufacturingtolerances, certain of the contact bumps are likely to be in a recessedposition relative to their neighbors and these recessed bumps will nothave a satisfactory opportunity to engage their pads since they will bedrawn away from their pads by the action of their neighbors on the rigidsupport. Furthermore, even when “scrub” movement is provided in themanner of Huff, the contacts will tend to frictionally cling to thedevice as they perform the scrubbing movement, that is, there will be atendency for the pads of the device to move in unison with the contactsso as to negate the effect of the contact movement. Whether anyscrubbing action actually occurs depends on how far the pads can move,which depends, in turn, on the degree of lateral play that exists as aresult of normal tolerance between the respective bearing surfaces ofthe probe head and chuck. Hence this form of membrane probing assemblydoes not ensure reliable electrical connection between each contact andpad.

A second conventional form of membrane probing assembly is exemplifiedby the device shown in Barsotti European Patent Pub. No. 304,868A2. Thisdevice provides a flexible backing for the central or contact-carryingportion of the flexible membrane. In Barsotti, the membrane is directlybacked by an elastomeric member and this member, in turn, is backed by arigid support so that minor height variations between the contacts orpads can be accommodated. It is also possible to use positive-pressureair, negative-pressure air, liquid or an unbacked elastomer to provideflexible backing for the membrane, as shown in Gangroth U.S. Pat. No.4,649,339, Ardezzone U.S. Pat. No. 4,636,772, Reed, Jr. et al. U.S. Pat.No. 3,596,228 and Okubo et al. U.S. Pat. No. 5,134,365, respectively.These alternative devices, however, do not afford sufficient pressurebetween the probing contacts and the device pads to reliably penetratethe oxides that form on the pad surfaces.

In this second form of membrane probing assembly, as indicated in Okubo,the contacts may be limited to movement along the Z-axis in order toprevent slippage and resulting misalignment between the contacts andpads during engagement. Thus, in Barsotti, the rigid support underlyingthe elastomeric member is fixed in position although it is also possibleto mount the support for Z-axis movement in the manner shown in HuffU.S. Pat. No. 4,980,637. Pad damage is likely to occur with this type ofdesign, however, because a certain amount of tilt is typically presentbetween the contacts and the device, and those contacts angled closestto the device will ordinarily develop much higher contact pressures thanthose which are angled away. The same problem arises with the relatedassembly shown in European Patent Pub. No. 230,348A2 to Garretson, eventhough in the Garretson device the characteristic of the elastomericmember is such as to urge the contacts into lateral movement when thosecontacts are placed into pressing engagement with their pads. Yetanother related assembly is shown in Evans U.S. Pat. No. 4,975,638 whichuses a pivotably mounted support for backing the elastomeric member soas to accommodate tilt between the contacts and the device. However, theEvans device is subject to the friction clinging problem alreadydescribed insofar as the pads of the device are likely to cling to thecontacts as the support pivots and causes the contacts to shiftlaterally.

Yet other forms of conventional membrane probing assemblies are shown inCrumly U.S. Pat. No. 5,395,253, Barsotti et al. U.S. Pat. No. 5,059,898and Evans et al. U.S. Pat. No. 4,975,638. In Crumly, the center portionof a stretchable membrane is resiliently biased to a fully stretchedcondition using a spring. When the contacts engage their respectivepads, the stretched center portion retracts against the spring to apartially relaxed condition so as to draw the contacts in radial scrubdirections toward the center of the membrane. In Barsotti, each row ofcontacts is supported by the end of a respective L-shaped arm so thatwhen the contacts in a row engage their respective pads, thecorresponding arm flexes upwardly and causes the row of contacts tolaterally scrub simultaneously across their respective pads. In bothCrumly and Barsotti, however, if any tilt is present between thecontacts and the device at the time of engagement, this tilt will causethe contacts angled closest to the device to scrub further than thoseangled further away. Moreover, the shorter contacts will be forced tomove in their scrub directions before they have had the opportunity toengage their respective pads due to the controlling scrub action oftheir neighboring contacts. A further disadvantage of the Crumly device,in particular, is that the contacts nearer to the center of the membranewill scrub less than those nearer to the periphery so that scrubeffectiveness will vary with contact position.

In Evans et al. U.S. Pat. No. 5,355,079 each contact constitutes aspring metal finger, and each finger is mounted so as to extend in acantilevered manner away from the underlying membrane at a predeterminedangle relative to the membrane. A similar configuration is shown inHiggins U.S. Pat. No. 5,521,518. It is difficult, however, to originallyposition these fingers so that they all terminate in a common plane,particularly if a high density pattern is required. Moreover, thesefingers are easily bent out of position during use and cannot easily berebent back to their original position. Hence, certain ones of thefingers are likely to touch down before other ones of the fingers, andscrub pressures and distances are likely to be different for differentfingers. Nor, in Evans at least, is there an adequate mechanism fortolerating a minor degree of tilt between the fingers and pads. AlthoughEvans suggests roughening the surface of each finger to improve thequality of electrical connection, this roughening can cause undueabrasion and damage to the pad surfaces. Yet a further disadvantage ofthe contact fingers shown in both Evans and Higgins is that such fingersare subject to fatigue and failure after a relatively low number of“touchdowns” or duty cycles due to repeated bending and stressing.

Referring to FIG. 1, Cascade Microtech, Inc. of Beaverton, Oreg. hasdeveloped a probe head 40 for mounting a membrane probing assembly 42.In order to measure the electrical performance of a particular die area44 included on the silicon wafer 46, the high-speed digital lines 48and/or shielded transmission lines 50 of the probe head are connected tothe input/output ports of the test instrumentation by a suitable cableassembly, and the chuck 51 which supports the wafer is moved in mutuallyperpendicular X,Y,Z directions in order to bring the pads of the diearea into pressing engagement with the contacts included on the lowercontacting portion of the membrane probing assembly.

The probe head 40 includes a probe card 52 on which the data/signallines 48 and 50 are arranged. Referring to FIGS. 2-3, the membraneprobing assembly 42 includes a support element 54 formed ofincompressible material such as a hard polymer. This element isdetachably connected to the upper side of the probe card by four Allenscrews 56 and corresponding nuts 58 (each screw passes through arespective attachment arm 60 of the support element, and a separatebacking element 62 evenly distributes the clamping pressure of thescrews over the entire back side of the supporting element). Inaccordance with this detachable connection, different probing assemblieshaving different contact arrangements can be quickly substituted foreach other as needed for probing different devices.

Referring to FIGS. 3-4, the support element 54 includes a rearward baseportion 64 to which the attachment arms 60 are integrally joined. Alsoincluded on the support element 54 is a forward support or plunger 66that projects outwardly from the flat base portion. This forward supporthas angled sides 68 that converge toward a flat support surface 70 so asto give the forward support the shape of a truncated pyramid. Referringalso to FIG. 2, a flexible membrane assembly 72 is attached to thesupport after being aligned by means of alignment pins 74 included onthe base portion. This flexible membrane assembly is formed by one ormore plies of insulative sheeting such as KAPTON™ sold by E.I. Du Pontde Nemours or other polyimide film, and flexible conductive layers orstrips are provided between or on these plies to form the data/signallines 76.

When the support element 54 is mounted on the upper side of the probecard 52 as shown in FIG. 3, the forward support 66 protrudes through acentral opening 78 in the probe card so as to present the contacts whichare arranged on a central region 80 of the flexible membrane assembly insuitable position for pressing engagement with the pads of the deviceunder test. Referring to FIG. 2, the membrane assembly includes radiallyextending arm segments 82 that are separated by inwardly curving edges84 that give the assembly the shape of a formed cross, and thesesegments extend in an inclined manner along the angled sides 68 therebyclearing any upright components surrounding the pads. A series ofcontact pads 86 terminate the data/signal lines 76 so that when thesupport element is mounted, these pads electrically engage correspondingtermination pads provided on the upper side of the probe card so thatthe data/signal lines 48 on the probe card are electrically connected tothe contacts on the central region.

A feature of the probing assembly 42 is its capability for probing asomewhat dense arrangement of contact pads over a large number ofcontact cycles in a manner that provides generally reliable electricalconnection between the contacts and pads in each cycle despite oxidebuildup on the pads. This capability is a function of the constructionof the support element 54, the flexible membrane assembly 72 and theirmanner of interconnection. In particular, the membrane assembly is soconstructed and connected to the support element that the contacts onthe membrane assembly preferably wipe or scrub, in a locally controlledmanner, laterally across the pads when brought into pressing engagementwith these pads. The preferred mechanism for producing this scrubbingaction is described in connection with the construction andinterconnection of a preferred membrane assembly 72 a as best depictedin FIGS. 6 and 7 a-7 b.

FIG. 6 shows an enlarged view of the central region 80 a of the membraneassembly 72 a. In this embodiment, the contacts 88 are arranged in asquare-like pattern suitable for engagement with a square-likearrangement of pads. Referring also to FIG. 7 a, which represents asectional view taken along lines 7 a-7 a in FIG. 6, each contactcomprises a relatively thick rigid beam 90 at one end of which is formeda rigid contact bump 92. The contact bump includes thereon a contactingportion 93 which comprises a nub of rhodium fused to the contact bump.Using electroplating, each beam is formed in an overlapping connectionwith the end of a flexible conductive trace 76 a to form a jointtherewith. This conductive trace in conjunction with a back-planeconductive layer 94 effectively provides a controlled impedancedata/signal line to the contact because its dimensions are establishedusing a photolithographic process. The backplane layer preferablyincludes openings therein to assist, for example, with gas ventingduring fabrication.

The membrane assembly is interconnected to the flat support surface 70by an interposed elastomeric layer 98, which layer is coextensive withthe support surface and can be formed by a silicone rubber compound suchas ELMER'S STICK-ALL™ made by the Borden Company or Sylgard 182 by DowCorning Corporation. This compound can be conveniently applied in apaste-like phase which hardens as it sets. The flat support surface, aspreviously mentioned, is made of incompressible material and ispreferably a hard dielectric such as polysulfone or glass.

In accordance with the above-described construction, when one of thecontacts 88 is brought into pressing engagement with a respective pad100, as indicated in FIG. 7 b, the resulting off-center force on therigid beam 90 and bump 92 structure causes the beam to pivot or tiltagainst the elastic recovery force provided by the elastomeric pad 98.This tilting motion is localized in the sense that a forward portion 102of the beam moves a greater distance toward the flat support surface 70than a rearward portion 104 of the same beam. The effect is such as todrive the contact into lateral scrubbing movement across the pad as isindicated in FIG. 7 b with a dashed-line and solid-line representationshowing the beginning and ending positions, respectively, of the contacton the pad. In this fashion, the insulative oxide buildup on each pad isremoved so as to ensure adequate contact-to-pad electrical connections.

FIG. 8 shows, in dashed line view, the relative positions of the contact88 and pad 100 at the moment of initial engagement or touchdown and, insolid-line view, these same elements after “overtravel” of the pad by adistance 106 in a vertical direction directly toward the flat supportsurface 70. As indicated, the distance 108 of lateral scrubbing movementis directly dependent on the vertical deflection of the contact 88 or,equivalently, on the overtravel distance 106 moved by the pad 100.Hence, since the overtravel distance for each contact on the centralregion 80 a will be substantially the same (with differences arisingfrom variations in contact height), the distance of lateral scrubbingmovement by each contact on the central region will be substantiallyuniform and will not, in particular, be affected by the relativeposition of each contact on the central region.

Because the elastomeric layer 98 is backed by the incompressible supportsurface 70, the elastomeric layer exerts a recovery force on eachtilting beam 90 and thus each contact 93 to maintain contact-to-padpressure during scrubbing. At the same time, the elastomeric layeraccommodates some height variations between the respective contacts.Thus, referring to FIG. 9 a, when a relatively shorter contact 88 a issituated between an immediately adjacent pair of relatively tallercontacts 88 b and these taller contacts are brought into engagement withtheir respective pads, then, as indicated in FIG. 9 b, deformation bythe elastomeric layer allows the smaller contact to be brought intoengagement with its pad after some further overtravel by the pads. Itwill be noted, in this example, that the tilting action of each contactis locally controlled, and the larger contacts are able, in particular,to tilt independently of the smaller contact so that the smaller contactis not urged into lateral movement until it has actually touched down onits pad.

Referring to FIGS. 10 and 11, the electroplating process to constructsuch a beam structure, as schematically shown in FIG. 8, includes theincompressible material 68 defining the support surface 70 and thesubstrate material attached thereon, such as the elastomeric layer 98.Using a flex circuit construction technique, the flexible conductivetrace 76 a is then patterned on a sacrificial substrate. Next, apolyimide layer 77 is patterned to cover the entire surface of thesacrificial substrate and of the traces 76 a, except for the desiredlocation of the beams 90 on a portion of the traces 76 a. The beams 90are then electroplated within the openings in the polyimide layer 77.Thereafter, a layer of photoresist 79 is patterned on both the surfaceof the polyimide 77 and beams 90 to leave openings for the desiredlocation of the contact bumps 92. The contact bumps 92 are thenelectroplated within the openings in the photoresist layer 79. Thephotoresist layer 79 is removed and a thicker photoresist layer 81 ispatterned to cover the exposed surfaces, except for the desiredlocations for the contacting portions 93. The contacting portions 93 arethen electroplated within the openings in the photoresist layer 81. Thephotoresist layer 81 is then removed. The sacrificial substrate layer isremoved and the remaining layers are attached to the elastomeric layer98. The resulting beams 90, contact bumps 92, and contacting portions93, as more accurately illustrated in FIG. 12, provides the independenttilting and scrubbing functions of the device.

Another suitable technique of the construction of a membrane probe isdisclosed in co-pending U.S. patent application, Ser. No. 09/115;571,incorporated by reference herein. However, for the inventions describedherein, the present inventors have no preference as to the particularconstruction of the contacting portion of the membrane assembly nor thegeneral structure of the membrane or membrane assembly itself.

While providing an improved technique for effective scrubbing action issignificant, the present inventors determined that excessive noise stillremains in the signals sensed by the measurement device.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks of theprior art by providing a membrane probing assembly with a probe cardthat includes conductors supported thereon, wherein the conductorsinclude at least a signal conductor located between a pair of spacedapart guard conductors. A membrane assembly includes a membrane withcontacts thereon, and supporting at least a signal conductor locatedbetween a pair of spaced apart guard conductors. The guard conductors ofthe probe card are electrically interconnected proximate theinterconnection between the probe card and the membrane assembly. Theguard conductors of the membrane assembly are electricallyinterconnected proximate the interconnection between the probe card andthe membrane assembly.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a membrane probing assembly bolted to aprobe head and a wafer supported on a chuck in suitable position forprobing by this assembly.

FIG. 2 is a bottom elevational view showing various parts of the probingassembly of FIG. 1, including a support element and flexible membraneassembly, and a fragmentary view of a probe card having data/signallines connected with corresponding lines on the membrane assembly.

FIG. 3 is a side elevational view of the membrane probing assembly ofFIG. 1 where a portion of the membrane assembly has been cut away toexpose hidden portions of the support element.

FIG. 4 is a top elevational view of an exemplary support element.

FIGS. 5 a-5 b are schematic side elevational views illustrating how thesupport element and membrane assembly are capable of tilting to matchthe orientation of the device under test.

FIG. 6 is an enlarged top elevational view of the central region of theconstruction of the membrane assembly of FIG. 2.

FIGS. 7 a-7 b are sectional views taken along lines 7 a-7 a in FIG. 6first showing a contact before touchdown and then showing the samecontact after touchdown and scrub movement across its respective pad.

FIG. 8 is a schematic side view showing, in dashed-line representation,the contact of FIGS. 7 a-7 b at the moment of initial touchdown and, insolid-line representation, the same contact after further verticalovertravel by the pad.

FIGS. 9 a and 9 b illustrate the deformation of the elastomeric layer tobring the contacts into contact with its pad.

FIG. 10 is a longitudinal sectional view of the device of FIG. 8.

FIG. 11 is a cross sectional view of the device of FIG. 8.

FIG. 12 is a more accurate pictorial view of the device shown in FIGS.10 and 11.

FIG. 13 is partial plan view of a membrane assembly and a probe card.

FIG. 14A is a partial pictorial view of the traces of a membraneassembly.

FIG. 14B is a partial plan view of the interconnection between amembrane assembly and a probe card.

FIG. 14C is a partial sectional side view of the interconnection betweenthe membrane assembly and the probe card of FIG. 14B.

FIG. 15 is a partial sectional view of a probe card illustrating theleakage currents from the end portions of the signal and guardconductors.

FIG. 16 is a partial sectional view of a probe card illustrating theinterconnecting of a pair of guard conductors together with a signalconductor therebetween.

FIG. 17 is a partial plan view of a portion of a probe card illustratingpower conductors, signal conductors, force conductors, sense conductors,removed interconnecting portions, and interconnected guard conductors.

FIG. 18 is a partial plan view of a portion of a membrane assemblyillustrating signal conductors, force conductors, sense conductors, andinterconnected guard conductors.

FIG. 19 is a partial plan view of a probe card and a membrane assemblysuitable for a Kelvin connection.

FIG. 20 is a partial plan view of a probe card illustrating differentgeometries for the interconnection to a membrane assembly.

FIG. 21 is a partial plan view of a membrane assembly illustrating aguard conductor looping around a respective probing device.

FIG. 22 is a plan view of a “pogo-pin” probe card constructed inaccordance with aspects of the present invention, where the connectionsto the probe card are normally electrical contacts from a probe aheadpositioned above the probe card.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With particular regard to probe cards that are specially adapted for usein measuring ultra-low currents, probe card designers have beenconcerned with developing techniques for controlling(e.g., minimizing)leakage currents. Unwanted currents that flow into a particular cable(or conductor)from surrounding cables (or conductors) may distort thecurrent measured in that particular cable (or conductor). For a givenpotential difference between two spaced apart conductors, the amount ofleakage current that will flow between them will vary depending upon thevolume resistivity of the insulating material that separates theconductors. In other words, if a relatively lower-resistance insulatoris used, this will result in a relatively higher leakage current. Thus,a designer of low-current probe cards will normally avoid the use ofrubber-insulated single-core wires on a glass-epoxy board since rubberand glass-epoxy materials are known to be relatively low-resistanceinsulators through which relatively large leakage currents can flow.

One technique that has been used for suppressing inter-channel leakagecurrents is positioning the signal conductor between a pair of guardconductors, where each guard conductor is maintained at the samepotential as the signal conductor by a feedback circuit in the outputchannel of the test instrument. Because the voltage potentials of theguard conductors and the respective signal conductor are made tosubstantially track each other, negligible leakage current will flowfrom the signal conductor to the corresponding guard conductors.Although leakage current can still flow between different sets of guardconductors, this is typically not a problem because the guardconductors, unlike the signal conductors, are at low impedance. By usingthis guarding technique, significant improvements may be realized in thelow-level current measuring capability of certain probe card designs byreducing the capacitance between signal and guard, and increasing theresistance between signal and guard.

To further improve low-current measurement capability, the membraneassembly is constructed so as to likewise minimize leakage currentsbetween the individual probing devices. Typically, this minimizationinvolves the selection of membrane materials and likewise providinglimited guarding of the signal conductor by a pair of guard conductorsto a location proximate the probing device. Referring to FIG. 13, toprovide the guarded path to a location proximate the probing deviceseach respective signal conductor 200 is located between a pair ofrespective guard conductors 202, 204 on the probe card 52, and themembrane assembly 72 likewise has a matching set of signal conductors206 and guard conductors 208, 210. It is thought that this arrangementprovides continuous sets of signal conductor/guard conductors to alocation proximate the probing devices in a manner to achieve lowleakage along nearly its entire length. However, even with the guardingof the signal conductors on the probe card 52 and the membrane assembly72, the leakage current levels remain unacceptable for low-currentlow-noise measurements.

In other probe card designs, efforts have been directed towardsystematically eliminating low-resistance leakage paths within the probecard and toward designing extensive and elaborate guarding structures tosurround the signal conductors along the signal path. For example, inone design, the entire glass-epoxy main board is replaced with a boardof ceramic material which presents a relatively high resistance toleakage currents. However, the ceramic material used in these newerdesigns is relatively more expensive than the glass-epoxy material itreplaces. Another problem with ceramic materials is that they arerelatively susceptible to the absorption of surface contaminants such ascan be deposited by the skin during handling of the probe card. Thesecontaminants can decrease the surface resistivity of the ceramicmaterial to a sufficient extent as to produce a substantial increase inthe leakage current levels. In addition, the more extensive andelaborate guarding structures that are used in these newer designs hascontributed to a large increase in design and assembly costs.

It should be noted that there are other factors unrelated to design thatcan influence whether or not the potential of a particular probe cardfor measuring low-level currents will be fully realized. For example, ifless special care is taken in assembling the probe card, it is possiblefor surface contaminants, such as oils and salts from the skin orresidues left by solder flux, to contaminate the surface of the card andto degrade its performance (due to their ionic character, suchcontaminants can produce undesirable characteristics). Furthermore, evenassuming that the card is designed and assembled properly, the card maynot be suitably connected to the test instrument or the instrument maynot be properly calibrated so as to completely null out, for example,the effects of voltage and current offsets. The probe card or theinterconnecting lines can serve as pickup sites for ac fields, which acfields can be rectified by the input circuit of the test instrument soas to cause errors in the indicated dc values. Thus, it is necessary toemploy proper shielding procedures for (1) the probe card, (2) theinterconnecting lines, and (3) the test instrument in order to shieldout these field disturbances. Due to these factors, when a new probecard design is being tested, it can be extremely difficult to isolatethe causes of undesirable background current in the new design due tonumerous and possibly interacting factors that may be responsible.

The present inventors reconsidered a seemingly improbable source ofnoise, namely, the interconnection between the probe card 52 and themembrane assembly 72, which from initial considerations would appear tobe effective at providing a guarded signal path to the probe devicebecause of the “continuous” signal path upon interconnection. However,upon further consideration the present inventors determined that thereis in fact significant unguarded and/or unshielded leakage pathsexisting in the region proximate the interconnection. Referring to FIG.14A, each conductive path of the membrane is normally encapsulatedwithin at least one layer of material (FIG. 14A illustrates multipleconductive paths without additional membrane materials). This provides astructure for routing conductive paths, such as the signal and guardconductors, to a location proximate the probing device without being onthe exterior (lower surface) of the membrane assembly which may resultin inadvertent contact with the device under test. Referring to FIGS.14B-14C, the signal and guard lines are actually interconnected betweenthe probe card 52 and the membrane assembly 72 by conductive structures220 that pass through the outer layer 222 of the membrane assembly 72 tothe interior conductive paths 206, 208, 210 of the membrane assembly 72.To form the electrical connection, the probe card 52 and membraneassembly 72 are mechanically aligned, and accordingly respectiveconductive structures 220 of the membrane assembly 72 are interconnectedwith the conductors 200, 202, 204 of the probe card 52. It is normallyundesirable for the membrane assembly 72 interconnection to electricallyconnect at the absolute end of the conductors 200, 202, 204 (e.g.,signal conductor and guard conductors) of the probe card 52 because thenthe tolerances for the interconnection would be extremely small,requiring nearly perfect alignment and extremely accurate fabrication.Accordingly, normally the signal and guard conductors supported by theprobe card 52 extend beyond the region of electrical interconnection.

After further consideration, the present inventors came the realizationthat this extension of the signal and/or guard conductors beyond thelocation of electrical connection results in significant additionalleakage paths. Referring to FIG. 15, the region 216 beyond the end ofthe guard conductors provides for surface leakage paths 218, which areprimarily DC in nature with the characteristic of an added resistancebetween the respective conductive paths. This surface leakage path froma signal conductor around the end of the adjacent guard conductorsreduces the accuracy of measurements by increasing the leakage currents.Also, the present inventors likewise realized that the region 216 beyondthe end of the guard conductors provides for a bulk leakage path, whichis primarily AC (e.g., not DC) in nature with the characteristic of anadded capacitance, between the signal conductor and the conductorsbeyond the adjacent guard conductors. This bulk leakage path from thesignal conductor around the end of the adjacent guard conductors reducesthe accuracy of measurements by increasing the leakage currents. It isto be noted that the guard conductors, in effect, impose a guard voltageinto the bulk of the probe card in a region generally underneath therespective guard conductor. This reduces the bulk capacitive leakagecurrents from the interposed signal conductor in regions with anadjacent guard conductor.

In many embodiments, the opening 230 into which the membrane assembly 72is supported includes a conductive surface 232 therein (e.g., guard,shield, ground) to further isolate the membrane assembly 72 from theprobe card 52. Unfortunately, the conductive surface 232 results insignificant fringe fields 234 (on the surface and in the bulk of theprobe card 52) at the end of the signal conductors 200 and guardconductors 202, 204. These fringe fields 234 appear to the measuringdevice as an additional parallel capacitance and resistance. This fringeleakage path at the end of the guard and signal conductors 200, 202, 204reduces the accuracy of measurements by increasing the leakage currents.The cumulative result of the additional bulk leakage currents,additional surface leakage currents, and additional fringe capacitanceand resistance (leakage currents), appears to the measuring device as acapacitance and resistance lumped together with the measurements of theactual device under test. It is difficult, if not nearly impossible, tocalibrate such additional leakage currents out of any measurements sothat the true measurement of the device under test is obtained. Further,the additional capacitance results in an increase in the settling timeof signals thereby increasing the time required to obtain a set ofaccurate measurements.

It is desirable to maximize the number of interconnections availablebetween the probe card 52 and the membrane assembly 72 in order toprovide the capability of probing an increasingly greater number ofdevices under test. While increasing the size of the membrane assembly72 to provide a greater circumferential edge may be employed, it remainsdesirable to limit the size of the membrane assembly 72 to minimize thelength of the conductive paths to reduce leakage currents.

To increase the number of interconnections available between themembrane assembly 72 and the probe card 52, the width of the conductorsof the membrane assembly 72 and the probe card 52 may be decreasedtogether with the spacing between the conductors. While decreasing thesize of the conductor increases the number of interconnections for agiven circumferential edge, this unfortunately results in an increaseddifficultly of aligning the respective conductive traces together.Further, the greater density increases the manufacturing expense of thedevice.

In general, the membrane assembly 72 is suitable for a higher density ofconductive paths than the probe card 52. Accordingly, the initial limitto the number of interconnects is the ability to fabricate anincreasingly greater number of conductive traces on the probe card 52.

Referring to FIG. 16 the present inventors came to the realization thatthe preferred solution to overcome the aforementioned drawbacks of thepresently accepted techniques is to interconnect the guard conductorsaround the end of the signal conductor, in contrast to the apparentsolution of merely decreasing the feature size of the interconnects. Theinterconnecting portion 240 for each respective pair of guard conductors(effectively one electrical conductor) is preferably on the same plane,such as the top surface of the probe card 52, together with the guardconductors and signal conductors. The interconnecting portion 240reduces the surface leakage path from the signal conductor byinterposing a guarded path around the end of the signal conductor. Inaddition, the interconnecting portion 240 likewise decreases the bulkleakage path from a signal conductor by imposing a guard voltage in aregion of the bulk of the probe card completely enclosing the end of thesignal conductor. Also, the fringe leakage path to the centralconductive surface 232 from the end of the signal conductor issubstantially reduced, or otherwise eliminated, by providing the guardedinterconnecting portion 240 around the end of the signal conductor.Reducing the leakage currents by including the additionalinterconnecting guard portion 240 results in the measurements made fromthe measuring device to be more accurate because less leakage currentsare erroneously included in the measurements. In addition, a decrease inthe settling time of the signals is achieved which reduces the timerequired to obtain a set of accurate measurements. One or more of theaforementioned drawbacks and/or advantages may be present and/orachieved depending upon the particular device and implementation.

With the interconnecting portion 240 electrically interconnectingtogether a pair of guard conductors 202, 204 another benefit is moreachievable, namely, increasing the number of potential interconnections,without necessarily changing the size of the membrane assembly 72,without necessarily changing the geometry of the individual conductors,and without necessarily decreasing the spacing between adjacentconductors. Referring to FIG. 17, the contacting region 250 for thecontacts 220 of the membrane assembly 72 on the probe card 52 areprovided on at least one side of the interconnected guard conductor 202,204, 240. This permits easier alignment of the membrane assembly 72 andthe probe card 52. The width of the guard conductor on the sidegenerally opposite the contacting region may be considerably thinnerbecause there is no contact by the membrane assembly 72 with thatportion of the guard conductor. The different widths of the guardconductors proximate the end of the signal conductor permits a greaterdensity of conductors to be achieved, if desired, without decreasing themechanical tolerances required. A pair of contacts (one on either sideof the signal conductor) may be used, if desired. As a result, thedensity of the interconnect between the probe card 52 and the membraneassembly 72 is closer to the capability of the membrane assembly 72.

Referring to FIG. 18, to provide a single contact between the pair ofguard conductors on the probe card 52 and a respective pair of guardconductors of the membrane assembly 72, the guard conductors of themembrane assembly 72 preferably include an interconnecting guard portion260 with the inderdisposed signal conductor, in a manner similar to theinterconnecting guard portion 240. The interconnecting membrane guardportion 260 provides many of the same advantages as described above withrespect to the interconnecting probe guard portion 240. By including theinterconnecting membrane guard portion 260, only a single conductivestructure 220 needs to be provided between the membrane assembly 72 andthe probe card 52 for each set of guard conductors.

Ideally in a two lead conductor system a “true Kelvin” connection isconstructed. This involves using what is generally referred to as aforce signal and a sense signal. The signal conductor from one of thetwo conductors is considered the force conductor, while the signalconductor from the other of the two conductors is considered the senseconductor. The force conductor is brought into contact with a test padon the wafer. The force conductor is a low impedance connection, so acurrent is forced through the force conductor for testing purposes. Thesense conductor is a high impedance connection and is also brought intocontact with the same test pad (or a different test pad) on the wafer,preferably in close proximity to the sense conductor, in order to sensethe voltage. As such the current versus voltage characteristics of thetest device can be obtained using the force and sense conductors.

Referring to FIG. 19, one potential technique to achieve a Kelvinconnection with the membrane probing system is to design the probe card52 to include multiple sets of a force conductor, a sense conductor, anda corresponding pair of guard connectors on opposing sides of theforce/sense conductors (preferably with the interconnection portion).The membrane assembly 72 likewise includes corresponding sets of a forceconductor, a sense conductor, and guard conductors (preferably with theinterconnecting portion). This provides a potential technique forachieving a Kelvin connection but unfortunately this wastesinterconnection space on the probe card 52 in the event that a Kelvinconnection for any particular device under test is not necessary.Alternatively, the probe card 52 may be redesigned for each membraneprobing assembly, which is typically unique for each application.However, redesigning the probe card 52 for each application is expensiveand not generally an acceptable solution.

While considering how to maintain one or more standard probe cards 52,together with providing Kelvin connections for each line, the presentinventors initially observed that the probe card 52 has more availablesurface area for routing the conductors further from the interconnectionbetween the probe card 52 and the membrane assembly 72. With theadditional surface area at regions not proximate the interconnectionbetween the probe card 52 and the membrane assembly 72, a pair ofconductive traces 280, 282 are easily routed, the pair being locatedbetween a pair of guard conductors 284, 286, to a location generallyproximate the interconnection (see FIG. 17). For non-Kelvinmeasurements, one of the conductors may be used as the signal line withthe remaining interconnected conductor not used. If desired, theinterconnection 270 between the two interconnected signal conductors maybe removed (open-circuited) for low noise measurements. However, withthe two signal conductors (e.g. force and sense) normally interconnectedit is a simple matter to break the interconnection 270 by removing aportion of conductors at region 290. In the event of “quasi-Kelvin”connections, the interconnection portion may be maintained and one ofthe pair of conductors 280, 282 would be used as a force conductor andthe other conductor of the pair would be used as a sense conductor.Quasi-Kelvin connections are generally formed by the interconnection ofa sense conductor and a force conductor at a point before the deviceunder test.

To accomplish effective probing with the membrane assembly 72, typicallylow impedance power conductors 300 are provided on the probe card 52 tosupply power to the probing devices of the membrane assembly 72. Thepresent inventors determined that the interconnection 270 between thepair of conductors may be removed and the force conductor 280 may bejumpered with a wire bond 302 (or any other suitable technique) to anunused power conductor on the probe card 52. Each of the powerconductors 300 on the probe card 52 are preferably conductive memberswithin the bulk of the probe card 52, electrically connected to thesurface of the probe card 52 by using a set of vias 304, 306. Each powerconductor is routed to a location proximate the interconnection betweenthe probe card 52 and the membrane assembly 72. The power conductor isnormally a low impedance conductor. Because the force conductor is alow-impedance connection designed to carry significant current it ispreferable to locate the force conductor outside of the guards 284, 286of its corresponding sense conductor. In addition, because the forceconductor is a low-impedance path carrying significant (non-negligible)current levels it does not necessarily require the guarding provided bythe guard conductors 284, 286 on opposing sides of the sense conductor282.

The power conductors, to which force conductors may be interconnectedwith, are preferably routed within the bulk of the probe card 52 in aregion directly underneath the corresponding sense conductor. Theconductive power conductor provides additional protection for the senseconductor to minimize leakage currents. Alternatively, the powerconductor may be routed on the top surface (or bottom surface) of theprobe card, if desired.

The power conductor is preferably routed to a point “interior” to theend of the corresponding signal conductor using a “via” 306 to the uppersurface of the probe card 52. Accordingly, the power conductor isavailable at a location suitable for interconnection to the membraneassembly 72, if desired, while likewise being available forinterconnection as a force conductor. In this manner, the same powerconductor may be used to provide power to the device under test, whilelikewise providing a force connection, both of which in a manner thatmaintains the density of the interconnection Of the interface betweenthe probe card 52 and the membrane assembly 72. The actual use of thepower conductors depends on the application and the particular design ofthe membrane assembly 72.

Another technique suitable to provide a greater density ofinterconnects, and their corresponding interconnecting regions (normallyhaving a greater surface area for contact to the membrane assembly 72)is to align the interconnects of the probe card 52 in a non-linearfashion (e.g., some closer and some farther from the edge of the probecard 52) around the circumference of the membrane assembly 72, as shownin FIG. 20. A further technique suitable to provide a greater density ofinterconnects, is to align the interconnecting regions in an overlappingmanner with respect to a direction perpendicular to the adjacentmembrane assembly 72. The membrane assembly 72 would likewise havecorresponding structures suitable to interconnect to the two-dimensionalstructure of the conductors of the probe card 52.

The present inventors came to the realization that the membrane assemblyis susceptible to absorption of moisture which increases the leakagecurrents within the membrane assembly. Referring to FIG. 21, anotherstructure suitable to reduce leakage currents for the probing devices isshown. Preferably, the guarded conductors 310 of the membrane assembly72 encircle the end of the probing device 312, with the signal conductorconnected thereto 314. Preferably, the guarded conductors 310 are withinthe bulk of the membrane assembly 72 to prevent their inadvertentcontact with the device under test. Providing the guarded probingdevices significantly reduces the effects of leakage currents betweenthe probing devices, especially due to the effects of humidity. However,the present inventors determined that the surface leakage currentsbetween adjacent probing devices may be reduced by removing at least aportion of the membrane material (dielectric) 316 in a locationproximate a portion of the guard conductors 310 and between the probingdevices 312. In this manner, a portion of the guard conductor 310 willbe exposed to the surface, albeit somewhat recessed from the surface ofthe membrane assembly 72, thereby impeding the passage of surfaceleakage between probing devices 312.

Referring to FIG. 22, in one embodiment of the present invention a pogopin probe card includes guarded signal paths and is suitable forreceiving a progo pin probe head for connection thereto.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A membrane probing assembly comprising: (a) a probe card with conductors supported thereon, wherein said conductors include at least a signal conductor located between a pair of spaced apart guard conductors; (b) a membrane assembly including a membrane with contacts thereon, and supporting at least a signal conductor located between a pair of spaced apart guard conductors; (c) said guard conductors of said probe card are electrically interconnected proximate the interconnection between said probe card and said membrane assembly; and (d) said guard conductors of said membrane assembly are electrically interconnected proximate the interconnection between said probe card and said membrane assembly. 